Semiconductor memory devices can be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include static random access memory (SRAM), dynamic random access memory (DRAM), and synchronous dynamic random access memory (SDRAM). Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), flash memory, phase-change random access memory (PRAM), resistive read only memory (RRAM), magnetoresistive random access memory (MRAM), and ferroelectric random access memory (FRAM).
Among nonvolatile memory devices, flash memory has achieved popularity in recent years due to attractive features such as relatively high storage capacity and performance, as well as low cost. Nevertheless, there is a continuing demand for flash memory devices having even higher storage capacity and performance, as well as lower cost. Accordingly, in an effort to address this demand, researchers are engaged in continual efforts to improve numerous aspects of flash memory.